7 CONVECTION

For the past 40 years, studies have shown the relative importance of air leakage (convection) over diffusion as a means of transporting moisture through building assemblies. Air convection is a function of air pressure difference and leakage area. The air pressure driving forces are established by wind, thermal buoyancy (warm air rising), and mechanical systems. Mechanical systems are the dominant force in most large buildings.

The impact of air convection or air leakage is that cavities in building assemblies tend to be filled with air of the same psychrometric conditions as the air on the source side. Cavity conditions could be described as potentially occupying all of the psychrometric conditions shown in figure 11, depending on several variables, including: (1) direction of air flow; (2) pressure drive; (3) leakage area; and (4) path through the cavity.

The best air pressure balance is neutral or minimal air pressure difference, where there is little air exchange across the assembly. Most museum buildings are slightly pressurized to control and filter the incoming fresh air stream. Positive air pressure in a humidified building during cold weather poses a certain risk to any leaking air cavities. Where there is no convection, or where there is no cavity, the psychrometric conditions may be very much like those described using standard profiling.

Given the importance of air convection, the exact values of vapor permeances of materials can be seen to be of minimal importance. Of greater importance is the integrity of the air barrier, which can protect cavities from damaging effects of air leakage into and out of the cavity. And of even greater importance is achieving effective control of the air pressure differences across the building envelope.